Unless otherwise indicated herein, the materials described in this section are not prior art to the claims and are not admitted to be prior art by inclusion in this section.
Many people use mobile stations, such as cell phones and personal digital assistants, to communicate with cellular wireless networks, which may provide communication services such as voice, text messaging, and packet-data communication to these mobile stations. The mobile stations and networks may communicate with each other over a radio frequency (RF) air interface according to an agreed air interface protocol, such as CDMA, LTE, WiMAX, GSM, WIFI, BLUETOOTH, and others now known or later developed.
Mobile stations may conduct these wireless communications with one or more base station systems, each of which may send communications to and receive communications from mobile stations over the air interface and may, together with associated network infrastructure, provide connectivity with one or more transport networks such as the public switched telephone network (PSTN) or the Internet for instance. With this arrangement, mobile stations may be able to communicate via a base station system with one or more remote endpoints such as other mobile stations or nodes accessible through one or more transport networks.
When mobile stations are powered on, reset, temporarily lose a signal, or for some other reason are seeking a connection to a network, they may be arranged to scan for wireless networks with reference to an internal table that is typically known as a preferred roaming list (PRL). For a given geographic region, the PRL may list wireless networks in a priority order, identifying each network by a system identifier (SID) and carrier frequency, among possibly other information. Each base station system in the region may then belong to a particular network and thus have an associated SID, and may broadcast that SID on one or more air interface control channels so that mobile stations scanning for a network may detect the SID and determine by reference to their PRL whether to connect with the base station system.
In practice, a mobile station may scan for coverage in descending order of network preference as indicated by the mobile station's PRL, until the mobile station successfully detects a base station system broadcasting a listed SID. At that point, the mobile station may then register to be served by the base station system and its associated network infrastructure. For instance, the mobile station may wirelessly transmit a radio access registration request message to the base station system, which may trigger update of a record in the network to indicate that the mobile station is operating within coverage of the base station system, and the mobile station may receive a registration response message acknowledging the registration.
Once a mobile station has registered with a base station system or is otherwise within coverage of a base station system, the mobile station may monitor control channel transmissions from the base station system in order to receive any relevant system information and any messages specifically addressed to the mobile station. For instance, the mobile station may monitor an air interface paging channel of the base station system to receive base station system parameters and page messages addressed to the mobile station. To conserve battery power, the mobile station may be arranged to carry out this monitoring process in a “slotted mode,” in which the mobile station generally operates in a low-power sleep state but wakes up (enters into a more full power/reception state) periodically to read particular timeslots of the paging channel. The base station system may then be arranged to transmit any information specifically for that mobile station, such as page messages, in one or more of those particular timeslots.
From time to time when a mobile station monitors control channel transmissions from the base station, the mobile station may read the SID of the base station as broadcast on a control channel, and the mobile station may determine from its PRL whether any higher priority networks exist in the region. If the mobile station determines from the PRL that one or more higher priority networks exist in the region, the mobile station may then repeat the system selection process described above and may thereby detect and register with a more preferred base station system. On the other hand, if the mobile station determines from the PRL that the SID of the currently serving base station system is the highest priority in the region, then the mobile station need not repeat the system selection process.